There's very little to do. I mean, how do you stop a 10 kilometer hunk of rock on a collision course for Earth? If they recognized it early enough, they might be able to send out a nuclear charge. It wouldn't blow up the rock at all, like was portrayed in Armageddon, but it would alter it's course just enough that the asteroid would miss Earth. Other than that, though, we're pretty much helpless. Almost any form of defense requires prior notice, and like I've said, that's something you rarely get with killer asteroids.